Red lead pigments have been used for many years to inhibit the corrosion of ferrous metals. The protective action of red lead pigments dispersed in an oil binder or vehicle, for example vegetable oils, are unexcelled. As is well known, red lead contains varying amounts of red lead oxide or tri-lead tetroxide (Pb.sub.3 O.sub.4) and litharge or lead monoxide (PbO). The amount of each of the oxides is dependent upon the control of the oxidation of metallic lead to litharge and the oxidation of litharge to red lead oxide during production of red lead pigments. Commercially, four grades of red lead pigment are made, namely 85%, 95%, 97% and 98% based upon the amount of red lead oxide in the pigment.
For many years red lead pigments were dispersed in raw linseed oil. While the primer paint thus produced had excellent corrosion inhibiting properties, the heavy lead pigments "settled-out" during storage. Then, too, the primer paint hardened in the container because of the reactivity of litharge, which is more reactive than red lead oxide, with the fatty acids in the raw linseed oil. In present day practice, the 85% grade red lead pigment is not generally used for primer paints which are to be stored for long periods of time because of the reactivity of litharge with raw linseed oil. Generally, the 95% grade pigment is used for primer paints which are to be stored for relatively short periods of time and the 97% and 98% grades of red lead pigment are used for primer paints which are to be stored for relatively long periods of time. The concentration of the red lead pigment in present day primer paints is about 12 pounds per gallon (1437.60 grams per liter) of paint solids. The primer paints are formulated to have a pigment volume concentration within the range of 30% to 40% and generally are made to the high side of the range, usually about 38%.
The particle size of the red lead pigment is dependent upon the mode of manufacture. Furnace red lead pigments, which are used to formulate commercial primer paints, have a relatively coarse particle size within the range of 2 to 10 microns. Heavily pigmented primer paints made with such furnace red lead pigments have good brushing characteristics. Fumed red lead pigments, on the other hand, have a relatively fine particle size within the range of 0.25 to 4 microns and are generally not used to make red lead primer paints because such primer paints have a fluffy consistency.
Recent developments in synthetic binders or vehicles, such as the alkyd resins, have resulted in primer paints having accelerated drying times and good penetrating and weathering characteristics. Short oil and medium oil alkyd resins are used because of their very short drying times. Long oil alkyd resins have longer drying times than either the short or medium oil alkyd resins and are usually not used in the formation of red lead primer paints. Because of the high reactivity of litharge with the synthetic alkyd resins, the higher grades of the furnace red lead pigments, that is, 97% and 98% grades are used to manufacture such primer paints. Fumed red lead is not used because of its greater reactivity.
As is well known, lead compounds are toxic, therefore all primer paints which contain red lead pigments are toxic. In recent years efforts have been directed to finding a suitable substitute for primer paints which do not contain any red lead pigments. However, primer paints containing red lead pigments continue to be the best inhibitors of corrosion in use today.